Engines of the above type have a spark plug for each of their cylinders, said spark plug being mounted on the cylinder head so that its electrodes are disposed inside the combustion chamber of the cylinder. Such a spark plug is fed with electricity in order to produce an ignition spark between its electrodes for each cylinder operating cycle.
Hitherto, such a spark plug has been fed with electricity from an electricity-producing source by making use of a transformer coil which receives low voltage electricity from the source and which is connected by a high tension cable to a mechanical distributor which is generally of the rotating wiper type and which is in turn connected by high tension cables to each of the spark plugs of the engine.
Such an installation is known firstly as being a source of electrical leakage due mainly to the length of high tension cable and to the presence of dampness and to deposits of various substances which encourage the retention of dampness, and secondly as emitting interference which spoils proper operation of radio installations.
The loss of energy due to said leaks must be compensated by using means capable of producing higher tension, thereby giving rise to even greater leakage.
In order to remedy some of the known drawbacks of the installations of the above type, proposals have been made to replace the mechanical distributor by an electrical distributor. Although this proposal avoids the drawbacks due to the rotating wiper and to electrical flashover between fixed points, it nevertheless fails to reduce the length of high tension cable between the various components of the installation.
Another prior art proposal has been to place electronic distribution on a low tension cable leading to a multi-outlet transformer group with its outlets connected by high tension cables to the various spark plugs.
This proposition omits the high tensions links between the coil and the distributor, but it retains the high tension cables which exist between the transformer group and the various ignition spark plugs.
The prior art also describes proposals for fitting an individual coil assembly on each spark plug. This applies, in particular, to the proposals in the following documents: DE-U-G 85 18 139.0, JP-A-57 193 776, JP-A-60 195 374, JP-A-51 137 011, JP-A-59 266 279, WO-A-86 04 480.
All of the proposals put forward in the above documents have the common feature of disposing the transformer assembly in a body which is extended by an end fitting or sleeve which is fitted over the porcelain stem of a spark plug. The consequence of this form of construction with the mass of the assembly being located at a head end is to impose reciprocating stresses on the stem due to vibration when the engine is running, and spark plug stems are not generally designed to withstand such mechanical forces. This gives rise to partial or total deterioration of the plug spark which is then no longer capable of performing its function.
All of these proposals also share the feature of not providing adequate electrical screening of the coil and the spark plug in order to reduce or eliminate the emission of radio interference. It also appears that these proposals are not suitable for providing sealing at the contact interfaces between the coils and the spark plug terminal pins.
The object of the present invention is to remedy the above drawbacks by proposing an integrated ignition-transformer assembly of particularly simple and reliable construction suitable for being individually associated with a conventional spark plug and requiring no major fundamental structural changes prior to being fitted to an existing heat engine.